(1) Field of the Invention
The present invention relates to a receiving apparatus having a waveform degradation compensating function, and more particularly to a technique suitable for compensation for waveform degradation an optical signal suffers due to an optical transmission line.
(2) Description of the Related Art
FIG. 20 is a block diagram showing one example of the existing optical transmission system. In FIG. 20, an optical transmission system 100 is made up of an optical transmitting apparatus 200, an optical amplification repeater 300, and an optical receiving apparatus 400, with an optical signal sent from the optical transmitting apparatus 200 being transmitted through an optical transmission line (optical fiber) 500 to the optical receiving apparatus 400 while being repeated/amplified properly in the optical repeater 300. Incidentally, although only one optical repeater 300 is illustrated in FIG. 20, naturally, two or more optical repeaters are employable, or no need therefore arises, depending on an optical signal transmission distance.
In general, in such an optical transmission system 100, for example, for a long-distance/large-capacity optical transmission represented by 10 Gb/s (giga bit/second) or 40 Gb/s, the waveform degradation of a received signal occurs remarkably due to “chromatic dispersion”, “polarization mode dispersion” or the like, which causes the deterioration of the transmission characteristic such as a reception sensitivity characteristic of an optical receiver 402 leading to the reduction of an aperture of an eye pattern (which will be referred to hereinafter as an “eye aperture”) of a received waveform so that limitation is imposed on transmission rate or transmission distance. In FIG. 20, reference numeral 403 designates a signal processing unit for receiving data and clock from the optical receiver 402 to conduct predetermined digital signal processing.
In this case, the “chromatic dispersion”, resulting from the material dispersion or the structure dispersion of an optical fiber itself, signifies a property that the propagation velocity in the interior of the optical fiber has a wavelength dependency. Concretely, the property is that the long-wavelength side around the zero dispersion wavelength delays while the short-wavelength side advances. The influence thereof depends upon the transmission distance, the characteristic of an optical fiber put to use, the variation of environmental temperature or the like and varies therewith.
On the other hand, the “polarization mode dispersion” signifies the property that, since the cross section of an optical fiber is not a true circle but having an elliptic configuration, two polarization principal axes (TE mode, TM mode) perpendicular to each other makes a difference in group velocity (propagation velocity) of an optical signal. The influence thereof depends upon the variation of stress applied to the optical fiber, the manufacturing condition and installation condition of the optical fiber, the variation of the environmental temperature or the like and varies therewith.
The transmission characteristic degradation stemming from the “polarization mode dispersion” has already been formulated, and the degradation degree P (dB) of the transmission characteristic due to the “polarization mode dispersion” is determined by an input power ratio γ (0<γ<1) of an optical signal with respect to two polarization principal axes of an optical transmission line 500 and the polarization dispersion value Δτ of the entire optical transmission line 500, and is given according to the following equation (1). In this connection, the influence thereof is in proportion to the square of the transmission rate and is in proportion to the root (square root) of the transmission distance.P∝γ(1−γ)Δτ2  (1)
In addition, so far, a dispersion shift fiber (DSF) in which the zero dispersion wavelength is shifted to 1.55 μm band has been used as the optical transmission line 500, or a dispersion compensation fiber (DCF) 401a designed to have a chromatic dispersion characteristic reverse to the chromatic dispersion has been located at the former stage of the optical receiver 402. This compensates for the waveform degradation stemming from the optical transmission line 500 to enlarge the eye aperture of a received waveform.
On the other hand, with respect to the degradation due to the “polarization mode dispersion”, a polarization maintaining fiber (PMF) 401b designed such that, when a linear polarization light is incident in a state where its polarization axis is aligned with the X axis (or the Y axis) of an optical fiber, the light propagates in the optical fiber in a state where the polarization state is maintained and only the X polarization light (or, the Y polarization light) is obtainable even at its outgoing end is placed at the former stage of the optical receiver 402. This reduces the “polarization mode dispersion” stemming from the optical transmission line 500.
However, in general, the degree of the wavelength degradation stemming from the “chromatic dispersion” increases with an increase in number of wavelengths to be multiplexed, and the degree of the waveform degradation originating from the “polarization mode dispersion” increases in proportion to the transmission rate or the transmission distance and, therefore, in the above-mentioned existing optical transmission system 100, there is a need to employ a different dispersion compensation fiber 401a or polarization maintaining fiber 401b for each optical transmission distance for setting up a waveform degradation degree range the optical receiver 402 permits.
This increases the number of kinds and using frequency of the needed dispersion compensation fiber 401a or polarization maintaining fiber 401b, thereby increasing the cost at the system construction or the management cost. Thus, this is unfeasible and lacks the extensibility (applicability) and the flexibility at the system reconstruction or the like.
Moreover, since the dispersion compensation fiber 401a or the polarization maintaining fiber 401b requires a high cost, and because of suffering from a large insertion loss, there is a need to employ a multistage configuration of optical amplification repeaters 300. Still moreover, in a recent very-long-distance/large-capacity optical transmission system represented by 10 Gb/s or 40 Gb/s, since the wavelength spacing becomes extremely short owing the WDM technique, difficulty is experienced in disregarding the variation of the wavelength characteristic caused by the above-mentioned environmental temperature variation.
For this reason, in the case of the very-high-rate optical transmission such as 10 Gb/s or 40 Gb/s, the mere employment of the dispersion compensation fiber 401a or the polarization maintaining fiber 401b does not reach the solution to the lack of flexibility of the system including cost and sufficient waveform degradation compensation of a received signal.